Differential sensing amplifier for content addressable memory
A differential sensing amplifier for content addressable memory is disclosed. In the differential sensing amplifier there is a detection circuit for detecting at an input node a change in an input signal comprising, a differential amplifier having a sense node and a reference node, a means for alternating the differential amplifier between a precharge phase and a sense phase, a precharge means for providing an input signal precharge voltage to the input signal via an input device, said input device selectively coupling the sense node to the input signal upon a change in the input signal, and a reference means for providing the reference node with a reference signal that continuously tracks the input precharge voltage during the precharge phase and actively maintains the input signal precharge voltage during the sense phase. In a specifically preferred embodiment the invention provides a sensing amplifier circuit for detecting a match or mismatch search condition on a high density content addressable memory matchline that is capable of rapidly sensing the capacitive matchline, limit the matchline voltage swing sensing all matchlines simultaneously, and providing the match and mismatch result for post processing, all under relatively low dynamic power while consuming little circuit area.
1. A content addressable memory comprising:
a matchline coupled to a row of content addressable memory cells;
searchlines coupled to a column of content addressable memory cells;
a sense amplifier having a reference node and a sense node for sensing a voltage level of the matchline after search date has been applied to the searchlines; and,
a decoupling device for decoupling the matchline from the sense node in response to a decoupling voltage.
2. The content addressable memory of claim 1 , further comprising a precharge circuit for precharging the matchline and the sense node to a precharge voltage.
3. The content addressable memory of claim 1 , wherein the decoupling device comprises an nmos transistor.
4. The content addressable memory of claim 3 , wherein the sense amplifier includes a differential amplifier, the differential amplifier comprising:
a first transistor coupled to the sense node;
a second transistor coupled to the reference node;
an output node for providing an output voltage level corresponding to the voltage level of the sense node relative to a reference voltage of the reference node, and
a third transistor for switching the differential amplifier between a precharge phase and a sense phase.
5. The content addressable memory of claim 4 , wherein the differential amplifier includes a latch for latching the output voltage level of the output node.
6. The content addressable memory of claim 5 , wherein the precharge circuit includes a first bias circuit for biasing the sense node.
7. The content addressable memory of claim 6 , wherein the first bias circuit includes a fourth transistor having a gate coupled to a bias voltage.
8. The content addressable memory of claim 7 , wherein the fourth transistor is a pmos transistor.
9. The content addressable memory of claim 8 , wherein the reference voltage is generated by a reference circuit having a fifth transistor serially connected to a tracking transistor.
10. The content addressable memory of claim 9 , wherein the fifth transistor has a gate for receiving the bias voltage.
11. The content addressable memory of claim 10 , wherein the gates of the decoupling device and the tracking transistor receive the decoupling voltage.
12. The content addressable memory of claim 11 , wherein the reference circuit includes reset means for turning off the reference circuit during the precharge phase.
13. A The content addressable memory of claim 11 , wherein said decoupling voltage is a decoupling bias voltage.
14. A method of operating a content addressable memory comprising:
(a) precharging a matchline to a precharge voltage level during a precharge phase;
(b) decoupling the matchline from a sense node of a sense amplifier in response to a decoupling voltage;
(c) driving the search data onto the searchlines;
(d) activating the sense amplifier for comparing the voltage level of the sense node to a reference voltage; and,
(e) changing an output voltage level of the sense amplifier when the matchline voltage level is different than the precharge voltage level.
15. The method of claim 14 , wherein the step of driving comprises activating a reference voltage generator for generating the reference voltage.
16. The method of claim 14 , wherein the step of changing further comprises latching the output voltage level.
17. The method of claim 14 , further comprising applying a bias current to the sense node for preventing the sense node from floating.
18. The method of claim 14 , wherein the step of precharging includes precharging the output voltage level of the sense amplifier to VDD.
19. The method claim 18 , wherein the step of activating the sense amplifier includes stopping precharge of the output voltage level.